The Lymphatic System:
The lymphatic circulatory system is a complex network of thin-walled vessels responsible for transporting lymphatic fluid collected from the interstitial tissues of the body back to the heart (Figure 1a) . Lymphatic fluid is propelled by both the intrinsic contraction and relaxation of lymphatic vessels and by external compressive forces of skeletal muscles.[1] Lymphatic fluid is composed of protein, fat, immune cells (primarily lymphocytes) and excess fluid collected from the soft tissues. Chyle is the fat and protein rich lymphatic fluid produced in the small intestines and makes up about 50% of the lymphatic fluid. Chyle contains high concentrations of long-chain triglycerides, lymphocytes, electrolytes, immunoglobulins, albumin, fibrinogen, glucose and fat-soluble vitamins.[2] Intestinal lymph drains to regional nodes and joins hepatic lymph through the cisterna chyli to feed into the thoracic duct. The thoracic duct is the end of the central lymphatic highway that transports lymphatic fluid to the left subclavian vein and back the blood circulatory system. The thoracic duct originates from the cisterna chyli around the level of the first and second lumbar vertebrae, lateral to the aorta (Figure 1a) . Lymph nodes function to regulate the content of lymph and can activate an immune response when triggered by a pathogen.